The invention relates to a powder for a magnetic recording medium and a process for manufacturing the same and, more particularly, to a dispersive powder suitable for a high density vertical magnetic recording medium.
Generally, a conventional magnetic recording is a horizontal recording that utilizes residual magnetism in parallel with the direction of the recording medium. The recording medium, for horizontal recordings, has a dendrite magnetic powder, such as .gamma.-iron oxide (.gamma.-Fe.sub.2 O.sub.3), chromium oxide (Cr.sub.2 O.sub.3) and .gamma.-iron oxide covered with a cobalt layer (.gamma.-Fe.sub.2 O.sub.3 -Co), as a magnetic layer on a base material in parallel orientation with the direction of the track of the recording medium. This kind of recording medium, however, has the defect that for a short wave signal the recording-reproducing characteristic of the recording medium deteriorates, because the recording medium tends to increase the self-demagnetizing field of the medium at high density recording. In order to successfully achieve high density recording without self-demagnetizing, it is necessary that the coercive force of the medium should be increased and the thickness of the magnetic layer should be thinner. However, at the present time, it is difficult to strengthen the coercive force of the magnetic layer, and it is undesirable to make the magnetic layer thinner, due to the resulting deterioration of the reproduced signal.
To avoid these problems and disadvantages, a vertical magnetic recording has been proposed for high density recording. Vertical magnetic recording uses the residual magnetism existing in a vertical direction relative to the track of the magnetic recording medium. This method of magnetic recording is suitable for high density recording, because as the recording density increases, the self-demagnetizing field decreases. For vertical magnetic recording, a recording medium having a magnetic layer of a cobalt-chromium alloy produced by sputtering is known; similarly, a recording medium produced by coating a base with a solution including a binder and a magnetic powder is known. In particular, the coating method uses a metal oxide magnetic powder having excellent corrosion and abrasion resistance, and surface flatness, as well as high productivity. Accordingly, the recording medium produced by the coating method is regarded as a dominant medium for use in vertical magnetic recording.
When the recording media has a coating layer of a magnetic powder, it is not necessary to orient entirely the magnetization axis for magnetization of the magnetic powder normal to the base of the recording media; but, good orientation of the magnetization axis for magnetization is still desirable. A hexagonal ferrite magnetic powder, such as barium ferrite, strontium ferrite, calcium ferrite, lead ferrite, and mixtures thereof, in solid solution, is easy to orient in a direction perpendicular to the surface of the recording media. These magnetic powders have hexagonal plate-like particles, and they have a magnetization axis for magnetization in a direction perpendicular to the surface of the particle. Accordingly, these magnetic powders are suitable for use as vertical magnetic recording media, because the surfaces of the powder particles are easily oriented by coating the base of the media with a solution of a binder mixed with the particles. The magnetization axis for magnetization of the particles is oriented perpendicularly to the surface of the recording media by either magnetic orientation or mechanical orientation treatment.
However, as these particles overlap each other because of their strong cohesion resulting from mixing the binder with the particles, it is difficult to disperse the particles uniformly in the coating solution. Consequently, the signal to noise characteristic of the recording medium made by coating the solution on the base is deteriorated.